Variable to target a text-input in my code.ts

I’m stuck. I’m no developer but trying to build a simple plugin. Since here it’s working, some bumps along the road, but there’s a tiny stone that it’s giving me a hard time. Basically, I have an input for saving a class (frame with some property applied), and after I save a class I need to clear that input in the UI. What’s the variable that targets the input element in my code.ts file? I’m trying to target using this code:

const input = document.getElementById("className") as HTMLInputElement;
input.value = '';

But when compiling it gives me errors both for the document and the HTMLInputElement.
What am I doing wrong? It should be quite simple to clear an input after saving something. Thanks for you help.